1. Field of the Invention
The present invention relates to a light emitting diode and further specifically relates to the light emitting diode wherein a light emitting element is provided on a metal wiring layer disposed on a substrate.
2. Description of the Related Art
Light emitting diodes (LEDs) have been used for mainly display, as a center of so-called indicators such as each kind of electronic equipment or measuring gauge.
In recent years, LEDs have a higher light emitting efficiency and there are less fear of heat generation. Therefore, light emission by flowing larger current than before has been enabled. Accordingly, use of the LEDs have been studied as light sources for smaller illumination, and a part of such LEDs is already put to practical use.
When the LED is used as the light source for illumination, luminance as large as dozens of times or more of that in a case of being used as a light source for an indicator, is necessary. Therefore, a method of securing the luminance by arranging a plurality of LED chips in an array, or a method of using a large-sized LED chip capable of flowing a large current, is used.
The aforementioned method of arranging the plurality of LED chips in an array, is used for the light source of green, red, and yellow, for example, in a traffic signal on a road. However, in this method, accurate mounting/wiring of several hundred LED chips is necessary to constitute the light source for one illumination, thus requiring much labor in manufacture and incurring higher cost.
In the method of using a large-sized LED chip that flows the large current as described above, high luminance is secured by increasing a size of 1 chip to a few millimeters square from a several hundred μm square in a case of a LED chip for display, and increasing a current to several A from several hundred mA in a case of the LED chip for display. In this method, the number of LED chips used in the light source for one illumination can be tremendously reduced, thus making it extremely easy to mount the LED chip. Focusing on such an advantage, study and development of enhancing an allowable limit value (so-called current capacity) of a quantity of current in one LED chip, has been progressed.
Further, the large-sized LED that flows the large current is considerably bright, if compared with the LED for display. However, brightness is not sufficient, if compared with a light bulb or a fluorescent light that are put to practical use as general illumination devices. Therefore, there is a request to realize LED capable of flowing a further large current in the future.
However, in order to increase the current that can be flown to the LED, a problem is not limited to the size of the LED chip which needs to be increased, and the following various problems are likely to be generated.
In a conventional LED, generally, circular electrodes are provided in a center of a surface of the LED chip. If the LED chip is increased in size, it becomes difficult to uniformly disperse and supply the current to overall LED chip from the circular electrodes. In order to uniformly disperse the current to the overall LED chip, a method of using branch-shaped wiring electrodes or transparent electrodes is known.
However, when the large current flows to the branch-shaped wiring electrodes to thereby increase a current density, breakage or deterioration occurs in the wiring electrodes by electromigration. The current that flows through such wiring electrodes are larger by two digits or three digits or more than the current that flows through an electric wire. The deterioration of the wiring electrodes is gradually progressed, thus damaging an advantage of the LEDs such that they have a long service life.
In order to prevent the breakage or deterioration of the wiring electrodes, it is effective to increase a cross-sectional area of the wiring electrodes. However, when widths of the wiring electrodes are increased to thereby increase the cross-sectional area, emitted lights are blocked by wide wiring electrodes, thus reducing a light extraction efficiency. Further, increase of heights (thicknesses) of the wiring electrodes can be considered. However, a formation process of the wiring electrodes becomes difficult and also the wiring electrodes are easily damaged when the LED chips are grasped by a collet, etc. When the wiring electrodes are damaged, an electric resistance of a damaged part becomes high, and there is a high possibility that the wiring electrodes are broken when the large current flows.
Further, a diameter of a bonding wire for a connection between the LED and a stem, needs to be thick to withstand the large current. Moreover, a stem side on which the LED chip is mounted, also needs to be formed to withstand the large current, and wiring formed of a thin film such as plating which can be utilized conventionally, can not withstand the large current.
Further, a power source for flowing the large current to the LED is necessary. However, in the power source capable of flowing the large current such as 5 A or 10 A, the cost is increased and sufficient insulation measure needs to be secured to prevent a generation of electric leakage.
In order to solve the above-described technical problem, patent documents 1 and 2 disclose a technique of connecting in series and arranging a plurality of LED chips for constituting one LED. By connecting the plurality of LED chips in series, the LED can be operated with high drive voltage and low drive current.
Further, in order to increase the light extraction efficiency of the LED, patent documents 3 and 4 disclose a technique of providing a metal light reflection layer between a semiconductor light emitting part and a substrate.    (Patent document 1) Japanese Patent Laid Open Publication No. 2008-72141    (Patent document 2) Japanese Patent Laid Open Publication No. 2008-211255    (Patent document 3) Japanese Patent Laid Open Publication No. 2009-200178    (Patent document 4) Japanese Patent Laid Open Publication No. 2008-263015
However, in the technique disclosed by the aforementioned patent documents 1 and 2, although epitaxial layers can be formed on an insulating substrate like 4 elements type LED, the epitaxial layers can not be formed on a transparent substrate. In a case of simply using a semi-insulating substrate, light emitted from an active layer toward a substrate is absorbed into a surface of the substrate, and therefore high light extraction efficiency can not be realized.
Further, in the patent documents 1 and 2, an air-bridge wiring is used in the wiring between LED chips. When the air-bridge wiring is made by wire bonding, it requires much labor and a higher cost. Therefore, in the patent documents 1 and 2, the air-bridge wiring is formed in a lump by using a photolithography. However, in this case, a sufficient cross-sectional area can be hardly obtained in wiring. When the sufficient cross-sectional area can not be obtained in the wiring, there is a higher risk of breaking the wiring by a stress of resin during resin molding. Further, in the air-bridge wiring, the wiring is set in a suspended state, and therefore efficient heat diffusion by heat conduction can not be performed, thus raising a temperature of the wiring and easily breaking the wiring due to electromigration. Therefore, a limit conduction current needs to be set to be low.